A critically ill patient treated in a hospital's intensive care ward usually receives nourishment parenterally or in liquid form directly in a vein. Since illness and injuries may change a patient's metabolism and energy consumption considerably from a standard consumption, dictated by the height, weight, age and sex, normally used as a basis of estimating the need of nourishment, a strong interest has arisen to employ measuring in order to find out the quantitative and qualitative need of nourishment of a critically ill patient. This is possible by means of so-called indirect calorimetry wherein, by measuring the exchange of respiratory gases, it is possible to calculate energy consumption on the basis of oxygen consumption in a certain state of equilibrium and to conclude the quality of a nourishment digested on the basis of a ratio of carbon dioxide output to oxygen consumption.
Most patients in an intensive care ward are connected to a respirator in order to maintain respiration and, thus, said gas exchange measuring should be effected by means of a device that can be connected to a respirator and is independent of the make and operating principle of a respirator.
As for the respirator manufacturers, at least Swedish companies Engstrom Medical Ab and Siemens-Elema have developed metabolism measuring instruments connectable to Engstrom Erica and Siemens Servo Ventilator 900 series, which instruments, however, partially use information obtained from the sensors of a respirator itself and, thus, such instruments can only be employed as an accessory to said equipment.
In terms of measuring technique, a particular problem has been flowmetry, which is needed, in addition to gas content measuring, in calculation of oxygen consumption and carbon dioxide output. The accuracy and reliability of flow sensors are put to a severe test in clinical conditions, particularly due to moisture and the polluting effect of secretions coming out of a patient.